Université de Montréal doctoral graduate Caroline Piaulet-Ghorayeb and her team are unequivocal: TRAPPIST-1 d, the third planet orbiting the small star TRAPPIST-1, doesn't have an Earth-like atmosphere.
Using the James Webb Space Telescope, they've observed that despite its Earth-like size and its position near the edge of its star's temperate zone, where liquid water could theoretically exist, this rocky planet is far from being an Earth 2.0.
"What we ultimately want to know is whether an environment like Earth's can exist elsewhere, and under what conditions," said Piaulet-Ghorayeb, now a postdoctoral researcher at the University of Chicago.
Operated by NASA, the European Space Agency and the Canadian Space Agency, the Webb telescope "is finally allowing us to ask that question for Earth-sized planets, and already, we can cross TRAPPIST-1 d off the list of worlds that might resemble our own," said Piaulet-Ghorayeb.
Her research, published today in The Astrophysical Journal, began as part of her PhD in astrophysics at UdeM's, where she worked at the Trottier Institute for Research on Exoplanets (IREx).
40 light-years away
The TRAPPIST-1 planetary system, a red dwarf star located 40 light-years from Earth, was unveiled in 2017. It holds the record for the largest number of Earth-sized rocky planets known to orbit a single star: seven in total.
TRAPPIST-1 is much dimmer and cooler than our sun. As a result, its temperate zone, the region where a planet might sustain liquid water on its surface, lies much closer to the star than in our solar system.
TRAPPIST-1 d lies on the cusp of that so-called "habitable" zone, yet is only two per cent of the distance that Earth is from the sun. TRAPPIST-1 d completes an entire orbit around its star every four Earth days.
Using the Webb telescope's Near-Infrared Spectrograph (NIRSpec), the team of astronomers did not detect key atmospheric molecules such as water vapour, methane, or carbon dioxide, gases that are abundant in Earth's atmosphere.
The researchers also ruled out a methane-rich atmosphere on TRAPPIST-1 d that would be similar to that of the rocky world Titan, Saturn's largest moon.
"There are a few reasons why we might not have seen signs of an atmosphere on TRAPPIST-1 d," said Piaulet-Ghorayeb.
"The planet could have an extremely thin atmosphere, like Mars, making it very hard to detect. It might also be shrouded in thick, high-altitude clouds, like Venus, which would block the signatures of certain gases. Or, it might simply have no atmosphere at all."
Known to be turbulent
It's not easy being a planet orbiting a red dwarf star like TRAPPIST-1. This star is known to be highly active, making it particularly challenging to study the planets in its system, something demonstrated in a 2023 study led by Olivia Lim, a PhD candidate at IREx. TRAPPIST-1 frequently emits powerful stellar flares, which can strip away the atmospheres of small nearby planets.
Despite these challenges, scientists continue to search for signs of atmospheres around the TRAPPIST-1 planets. Red dwarfs are the most common type of star in our galaxy, and if some planets can retain their atmospheres despite the harsh radiation from their host stars, it could open the door to the possibility of habitable environments elsewhere, especially in more favourable conditions.
"The Webb telescope's sensitive infrared instruments are allowing us, for the first time, to examine the atmospheres of these small, cooler planets," said UdeM affiliated professor Björn Benneke, an IREx member and co-author of the study. "We're just beginning to explore their atmospheres using Webb's advanced tools, trying to understand which planets can hold on to their atmospheres, and which ones can't."
More planets to explore
Observations with the Webb telescope are ongoing for TRAPPIST-1's outer planets: e, f, g, and h. These more distant worlds are promising targets, but they also present unique challenges for astronomers.
As Benneke pointed out, these planets are more likely to retain their atmospheres because they're farther from their active host star. Nevertheless, that same distance makes it harder for Webb's infrared instruments to detect any atmospheric signatures.
"We shouldn't lose hope of finding atmospheres around the other TRAPPIST-1 planets," said Piaulet-Ghorayeb.
"Even though we didn't find a strong atmospheric signal on TRAPPIST-1 d, it's still possible that the more distant planets contain water or other components in their atmospheres that could tell us something about their potential habitability."
About this study
"Strict limits on potential secondary atmospheres on the temperate rocky exo-Earth TRAPPIST-1 d," by Caroline Piaulet-Ghorayeb et al., was published Aug. 11, 2025 in The Astrophysical Journal. DOI: 10.3847/1538-4357/adf207. In addition to Caroline Piaulet-Ghorayeb and Björn Benneke, the research team includes Keavin Moore, Pierre-Alexis Roy, Olivia Lim, René Doyon, Loïc Albert, Michael Radica (now at University of Chicago), Louis-Philippe Coulombe, David Lafrenière, Nicolas B. Cowan, Alexandrine L'Heureux, Romain Allart, Lisa Dang, Stefan Pelletier (now at Université de Genève), and Jason F. Rowe from IREx; three students mentored by Caroline Piaulet-Ghorayeb through the InitiaSciences program; and six other co-authors based in Canada, the United States, the United Kingdom, Switzerland, and France.
